cglpk.write_lp is now write and also does mps

The format is detected from the file extension.
Saving gene names in JSON files; specified default JSON dump params.

Specifically, allow_nan=False.
Merge pull request #127 from zakandrewking/master

Save genes in JSON and mat files, and options for JSON export.
Parse formula containing floating point values

A warning is still raised when floats are encountered, but this does not
prevent parsing of the composition.

This fixes #128